I've tried looking for lower parts kits/ spare springs/ original equipment springs from Colt or DPMS at all the US retailers and they are sold out. Really all you need is a small enough spring to fit the hole, you can cut to length as well, I'd head to Staples and look at finding a small pen with a spring inside, or maybe Rona in the hardware dept, they have a ton of screws/springs/etc.
Or maybe a craft type store or model kit store that has those remote control cars, I could see springs being there.
If you lose the detent, I read that it can be replaced with a small piece of coat hanger wire, just cut a few millimeters off and shape to fit with dremel or sandpaper.